Optical communication has widely been put into practical use since a long time ago, particularly in the field of long-distance communication, due to its ability of transmission over long distance with large capacity. Semiconductor laser has generally been used as a light source of transmitter in optical communication.
It is desirable to suppress electric resistance of the semiconductor laser, because it increase power consumption and heat generation. The heat degrades characteristics and lifetime of the device, and may even degrade speed of modulation. In particular, VCSEL (Vertical Cavity Surface Emitting Laser), a kind of surface emitting laser, shows large resistivity due to its small areas of electrodes and active layer, and also shows large thermal resistivity. Therefore, influences of the heat generation become large, which limit output and modulation speed of the device.
On the other hand, semiconductor lasers having large length of resonator, such as excitation lasers, are relatively small in resistance, but are large in operation current and consequently large in heat generation, which causes saturation of output. Therefore, further reduction in the resistivity is desired. It is effective to increase area through which current can pass for reducing the resistivity. Thus, efforts have been made on reducing the resistivity by increasing width of current aperture or width of active layer stripe.
However for the case of VCSEL, increase in the width of current aperture generally reduces modulation bandwidth. It also leads to a multi-mode oscillation, which is not suitable for communication with single-mode fiber. Also for the case of edge-emitter-type laser, increase in the stripe width of active layer may undesirably result in the multi-mode oscillation.
To solve the problem, a method for reducing the resistivity by using carrier (electron-hole) inversion with tunnel junction was proposed. With the method, a most part of p-type semiconductor having high resistivity can be replaced with an n-type semiconductor.
Following current confinement structures with the tunnel junction have ever been proposed. (1) a structure based on combination of the tunnel junction and oxide confinement layer. The current confinement layer is formed over the tunnel junction by selective oxidizing of aluminum-rich layer (see Non-Patent Document 1, for example), (2) a selective tunnel junction destruction method. A part of the tunnel junction is destructed by electrode-composing metal diffusion under annealing (see Non-Patent Document 2, for example), and (3) a buried tunnel junction structure. A part of the tunnel junction is removed by etching, and then buried in a semiconductor layer or the like (see Non-Patent Document 3, for example).
Of these, the technique disclosed in Non-Patent Document 3 is excellent in controllability and reproducibility as compared with the other methods, because the current confinement width may be controlled relying upon accuracy in photolithography and etching.
The structure is also successful in suppressing strain and defect to lower levels as compared with the other methods, due to its semiconductor buried structure. Besides, optical confinement can be small, which leads to single mode oscillation with a relatively large current aperture.
[Non-Patent Document 1] Applied Physics Letters, vol. 71, p/3468 1997, J. J. Wierer et al.
[Non-Patent Document 2] IPRM'99 TuB1-4, S. Sekiguchi et al.
[Non-Patent Document 3] Laser Conf. 2000 ThC2, R. Shau et al.
[Non-Patent Document 4] Jpn. J. Appl. Phys., Vol. 39, No. 4A, pp. 1727-9 (2000), Ortsiefer et al.
[Patent Document 1] U.S. Pat. No. 6,515,308
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2003-298187